When a workpiece is processed in a stretch forming process with a traditional stretch forming machine, the stretching action is generally provided by means of only one or two hydraulic cylinders, and the sheet material of the workpiece is substantially integrally displaced in a transverse direction at the edge clamped by the clamps. When a workpiece having a relatively large transverse curvature is processed, the distribution of stretch stress and stretch strain in the transverse direction of the sheet material becomes obviously not uniform, which results in processing defects such as departure from mold, cracking and wrinkling. In order to avoid these defects, precision requirements to the forming machines are very high, and the control systems of them have generally complex designs, thus the forming machines are very expensive. In addition, the blank material for stretch forming has generally a relatively large non-processed margin area, which results in low material usage.